Platelets are on of the most troublesome problems during many operations on the human body, yet are one of the most vital at the end of such surgery. During surgery, such as heart-lung bypass procedures, platelets become activated leading to aggregate formation. These platelet aggregates can form embolli which lodge in the small vessels of the patient, especially the lungs. This leads to problems due to loss of blood flow through these vessels resulting in tissue death. In the lungs, such aggregates can bring on "shock lung syndrome" leading to impaired lung function and slower patient recovery. Additionally, the platelets try to carry out their mission of blood coagulation at a time when clotting may not be needed or desired. Such an instance is when it is required to have blood flow in foreign vessels such as blood tubing, oxygenators, and other such devices.
Platelets are often damaged and lost to the patient due to the aformentioned mechanisms. At the end of surgery when it is desired to have the patient's blood clot, his platelet count may be so reduced or platelets so traumatized that they are no longer of value in the clotting process. The patient suffers unnecessary bleeding, perhaps even leading to reoperative procedures. Often, foreign platelets are given to such patient.
It would be desirable to remove platelets from such patients prior to the actual operative procedure, store them in a quiescent environment, and then return them to the patient at the end of the procedure.
This invention relates to a novel system for accomplishing this feat.
When a mixture of platelets, red blood cells, white blood cells, and plasma passes through a conduit such as a hollow fiber, some of the platelets will migrate to the wall of the fiber. They may become caught in the boundary layer of plasma very near the wall of the fiber. This boundary layer formation is a commonly understood consequence of flow through conduits. It is an area of nearly zero flow velocity compared to the generally laminar parabolic flow velocity in the bulk flow. These boundary layers are quite thin. Platelets, because of their small size, may fit within these boundary layers and not be disturbed by the bulk flow including collisions with other larger particles. Platelets generally circulate into and out of the boundary layer, but may be found in slightly larger concentrations in the boundary layer due to the aforementioned mechanisms.
When the walls of such tubes are porous to the water in the plasma, water will pass through the pores due to the transmembrane pressure across the membraneous walls of the tube. This process is commonly called ultrafiltration. Water will flow out of the system in a path perpendicular to the flow velocity of the blood. This has two effects. First, the boundary layer is further thinned, and secondly, platelets and other particles will tend to be drawn to the walls of the tube. I have found that this is advantageous for platelet capture, since more platelets will visit the tube wall and the thinner boundary layer allows for even less disturbance from larger particles which do not so readily enter this area.
Conditions with regard to tube size, shear rate, ultrafiltration flux and surface material and structure can be optimized to augment my novel process. Generally, the opposite is sought since the main goal is typically ultrafiltration and such platelet attraction and capture result in polarization concentration which inhibits the water flux rate. As a result, such devices are generally designed with tube sizes or mixing effects to prevent such polarization. However, I have discovered that augmenting the polarization process with subsequent recovery operations offers a new way to capture and concentrate platelets. This same process could be used to capture other particles or combinations of particles.